Metal insulator semiconductor field effect transistor (“MISFET”) devices are contained in an integrated circuit as basic devices. A miniaturization technique is applied to a MISFET device to shorten its channel length in order to make an operation speed of the integrated circuit high. Since gate insulation and electrode films are also made thinner with the miniaturization, their materials conventionally used for the high speed operation are reaching limits. Thus, new materials, and new device structures and manufacturing methods in which such new materials are applied have been developed.
Polycrystalline silicon, for example, generally used for a gate electrode material is high in resistivity. Metal or silicide is employed instead. When such a material is applied to an integrated circuit consisting of complementary metal oxide semiconductor (“CMOS”) circuits, for example, gate electrodes of N-channel and P-channel MISFET devices are made from identical materials so that the work functions of both gate electrodes become equal. Thus, it is extremely difficult to control proper threshold voltages of the N-channel and P-channel MISFET devices required for circuit operations, respectively.
To avoid this difficulty, a method of controlling the work function has been proposed in Japanese Patent Disclosure 2001-20376, page 15, FIG. 1. In the method, TiN, for example, is used to form a gate electrode film and a composition rate of the TiN electrode for an N-channel MISFET is changed by implanting nitrogen ions into the electrode. The method can control properly threshold voltages of the N-channel and P-channel MISFET devices, respectively.
The proposed method, however, has unstable factors as a method of manufacturing semiconductor devices. Such unstable factors are that heating treatment required after forming the gate electrode film causes deterioration of a withstand voltage of the gate electrode and that the generation of interface states results in reduction of driving capability of the gate electrode.